The invention proceeds from an electronic operating device for gas discharge lamps in accordance with the preamble of claim 1. What is involved here is, in particular, switching topologies which reduce the potential of at least one output terminal (2, 3) with respect to the earth potential. Output terminals are to be understood as the terminals of an operating device to which lamps are connected. If no further details are given with reference to a potential, potentials are taken to be referred to earth potential.
The potentials of the output terminals of an electronic operating device for gas discharge lamps should be kept as small as possible for the following reasons: firstly, an insulating problem ensues if an output terminal is at an excessively high potential. Leakage currents toward the earth potential which are no longer tolerable can arise; the protection against touching of the devices may in some circumstances no longer be ensured. Since the level of the potential of an output terminal is also a safety-relevant variable, its root-mean-square value is limited by the IEC Standard 60928.
Secondly, at high frequencies a high potential of an output terminal causes severe cable-conducted common-mode interference on the power leads. The lower this potential is, the less must be expended on radio interference suppression.
It is assumed that the potential difference between two points is determined by r.m.s. measurement of the voltage between the points. It is permissible in the following considerations to start from two lamp connections which are connected to the output terminals (2, 3) of the operating device. Of the usual four connections for lamps with heated filaments, respectively two which are connected by one filament are at approximately the same potential. The lamp arc voltage required for the lamps to be operated is present between the two significant lamp connections. The higher potential of the two output terminals (2, 3) is critical with reference to the above-described problem. The problem is compounded with rising lamp arc voltage. In particular, modern low-pressure discharge lamps with a diameter of 16 mm have an increased lamp arc voltage by comparison with conventional lamps with 26 mm. In the case of operating devices for two lamps, it has so far not been possible to connect said 16 mm lamps in series, since in the case of series connection the doubled lamp arc voltage causes the potential of at least one output terminal to exceed permitted limited values with respect to the earth potential. The prior art for solving this problem is therefore to be seen in the parallel connection of the lamps. However, when connecting the lamps in parallel it is necessary to provide a lamp circuit for each lamp, and this raises the costs, the weight and the space requirement of the device.
It is the object of the present invention to provide an operating device in accordance with the preamble of claim 1, in which the maximum of the potentials of the output terminals is as low as possible.
In the case of an operating device having the features of the preamble of claim 1, this object is achieved by the features of the characterizing part of claim 1. Particularly advantageous refinements are to be found in the dependent claims.
An electronic operating device for gas discharge lamps generally has an AC voltage generator (G) which makes available an AC voltage which has a frequency which is substantially higher than the frequency of the system voltage. The AC voltage is present mostly in a unipolar fashion at a generator output (1) with reference to a reference potential (E) which is close to the earth potential. The AC voltage is fed into a reactance network (Z) in order to transform the source resistance of the AC voltage generator (G) to a value which is suitable for operating the lamp. This provides a first output terminal (2) at which one or more series-connected lamps are connected. The second output terminal (3) leads to the reference potential (E) via the coupling capacitor (CB). The coupling capacitor bears the DC voltage component of the AC voltage source so that the lamp is operated with AC voltage free from DC voltage. It is frequently required for the coupling capacitor (CB) to be connected to a terminal at the reference potential (E) so that the voltage present at it can be better used by other components of the operating device.
Thus, the potential of at least one output terminal (2, 3) must be reduced in accordance with the object of the invention formulated above. As a rule, the first output terminal (2) has a higher potential, for which reason the latter must first and foremost be reduced. This is performed in accordance with the invention by inserting serially into the connection between the second output terminal (3) and the reference potential (E) an electric component (VC) which acts as a voltage source. The reduction voltage (UVC) formed thereat must have a voltage profile which is suitable for reducing the potential of the first output terminal (2). This condition can be observed, for example, by providing the electric component as a controlled voltage source. The control is then to be set such that the frequency of the reduction voltage (UVC) is equal to the frequency of the AC voltage generated by the AC voltage generator (G). Said controlled voltage source can be implemented in a particularly cost-effective way by means of coupled coils.
Of course, the abovenamed measures must not allow the potential of the second output terminal (3) to exceed the potential of the first output terminal (2). Potentials of the output terminals (2, 3) are equal in the ideal case.